1. Field of the Invention
The present invention relates to a plasma treatment method and apparatus for treating a body, and more particularly to a plasma treatment method and apparatus for forming fine and deep trenches and holes in a semiconductor wafer.
2. Description of the Related Art
Both dry etching and wet etching techniques have been used in the past for treating semiconductor wafers. By comparison, however, dry etching techniques permit easier microfabrication, and they have thus found wider use in the manufacture of large-scale integrated circuits (LSIs).
An example of a conventional dry etching technique is the reactive ion etching (RIE) method, which requires a gas pressure of 10 mTorr to 100 mTorr, and a reaction gas flow of 10 sccm to 100 sccm for effective gas etching. In such an RIE method, a lower gas pressure than described results in unstable discharges, while a higher gas pressure than described results in an isotropic etching. Often, such systems include pumps for evacuating reactive gas at an evacuation rate of less than 1000 l/sec.
Japanese Laid-Open Publication No. 52-126174, and Samukawa, "Perfect Selective, Highly Anisotropic, and High Rate ECR Plasma Etching for N.sup.+ Poly-Si and WSi.sub.x /Poly-Si," Extended Abstracts of the 22nd (1990 International) Conference on Solved State Devices and Material, pages 207-210, disclose a microwave plasma etching (ECR) technique. RIE dry etching systems are described in Sekime, et al, "Highly Selective Etching of Phosphorus-Doped Polycrystalline Silicon at Low Wafer Temperature Employing Magnetron Plasma," 1988 Dry Process Symposium (pages 54-57), and Perry, "The Application of the Helicon Source to Plasma Processing," J. Vac. Sci. Technol., B9(2), March/April 1991 (pages 310-317). These dry etching machines provide a reaction gas pressure of more than 0.5 mTorr in a gas flow of less than 20 sccm. The etching speed achieved by these systems is about 300 nm/min in the case of ECR etching with a polycrystalline silicon, a chlorine gas as a reaction gas, a gas pressure of 0.5 mTorr and a gas flow of 20 sccm.
An illustrative example of a conventional dry etching machine of the microwave plasma type is shown in FIG. 16. Reference numeral 101 represents a microwave source, 102 a waveguide, 104 a gas inlet, 105 a gas pipe, 106 a gas flow rate controller, 107 a solenoid coil, 109 a wafer, 110 a sample stage and 111 a vacuum chamber, whose pressure is lowered by a vacuum pump 114. RF power supply 112 and discharge zone 117 are further shown.
Microwaves generated by the microwave source 101 are introduced through the waveguide 102 into the discharge zone 117, where the reaction gas is converted into plasma, which in turn etches the surface of the wafer 109 placed on the sample stage 110. In this dry etching system, one kind of gas is introduced into the discharge zone 117 through one gas pipe 105 and one gas flow rate controller 106. The gas pipe 105 is directly connected to the discharge zone 117. The area of the opening portion of the gas inlet 104 is about the same as the cross-sectional area of the gas pipe 105.
The gas pressure in the vacuum chamber 111 rises as the flow rate of the reaction gas increases, and lowers as the effective speed of evacuating the vacuum chamber 111 by the vacuum pump 114 increases. For a gas pressure of more than 1 mTorr, the gas flow rate is set at several tens of sccm; for a low gas pressure region at 0.1 mTorr, the flow rate is set at several sccm. The effective speed of exhaust is determined by the pumping speed of the vacuum pump 114 and the conductance of the evacuation system for the vacuum chamber 111. Conventional systems have an effective exhaust speed of 400 l/sec or less.
The pressure in the vacuum chamber is related to the gas flow as follows: EQU P=(q+Q)/S (1)
where P (in Torr) represents the pressure in the vacuum chamber, q represents the leak rate from the equipment when no gas is introduced, Q represents the flow rate of gas introduced (Torr.multidot.l/sec), and S represents the effective evacuation speed of the equipment (l/sec). Normally, q is less than 1/1000 of Q and almost negligibly small.
Conventional systems have a turbomolecular pump with an exhaust speed (S.sub.i) of about 1000 l/sec or less and an exhaust conductance of the vacuum chamber (C) of between 200 l/sec and 1000 l/sec. The effective speed of exhaust S for a system having n such turbomolecular pumps is given by: ##EQU1##
These conventional systems have been capable of performing evacuation at an effective exhaust speed of 100 to 400 l/sec. Hence, the flow rate of gas that can be achieved when the gas pressure is set at 0.5 mTorr is 4 to 20 sccm.
The residence time for a gas to stay in the vacuum chamber, which is a measure of ease with which the gas can flow in the vacuum chamber, is expressed by: ##EQU2## where V is the total volume of the vacuum chamber. In these conventional systems, since the effective gas exhaust rate is 100 to 400 l/sec and the vacuum chamber volume is 100 to 300 l, the gas residence time is about 400 msec to 3 sec.
As LSIs become smaller and smaller, there are growing demands for development of a fabrication technology to form grooves and holes of about 0.3 .mu.m in size. With dry etching systems using the conventional RIE, it is difficult to form fine grooves and holes with high precision because the high gas pressure causes scattering of ions in the gas plasma, disturbing the direction in which the ions strike the substrate.
By lowering the gas pressure in the vacuum chamber, it is possible to prevent the scattering of ions that are incident on the sample. To perform an anisotropic fabrication of grooves and holes of the above-mentioned size, it is necessary to limit the angle of incidence of ions striking the sample to less than 1.degree., and the gas pressure in the vacuum chamber to less than 1 mTorr, and preferably less than 0.5 mTorr. To keep the plasma discharges stable requires a pressure of at least 0.01 mTorr.
Such conventional dry etching systems having low gas pressures include ECR etching systems, magnetron discharge RIE systems and helical resonator discharge RIE systems. These conventional systems, however, have a problem in that lower gas pressure results in reduced etching rate. In other words, increasing the etching directionality and heightening the etching speed are two conflicting requirements that have been difficult to meet at the same time.
Meanwhile, diameters of silicon wafers for LSIs are increasing. The ECR etching systems of the prior art use a single wafer system in which wafers are carried to the vacuum chamber one at a time for etching With such equipment, it takes about one to two minutes to reduce the thickness of a 6-inch polysilicon wafer by 200 nm at an etching speed of 200 to 300 nm/min. When an 8-inch wafer is used, the etching speed decreases due to an etching area dependency (loading effect), prolonging the processing time to two and four minutes and deteriorating the throughput.
The throughput may be improved by raising the etching speed by increasing the input power of the high frequency or microwave source. However, this gives rise to another problem in that increased ion energy reduces selectivity. While it is impossible to improve throughput without changing the etching conditions by using a plurality of signal-wafer-system dry etching systems for parallel processing, the equipment cost becomes prohibitively high.
The above ECR etching systems have another drawback. The cross-sectional area of the opening portion of the gas inlet is small. When the effective gas exhaust rate is set higher than that of the conventional systems to increase the flow rate of gas flowing in the vacuum chamber 111 to more than 1300 l/sec, for example, the velocity of the gas flowing from the gas inlet 104 into the vacuum chamber 111 approaches the speed of sound, causing shock waves in the gas flow, which in turn makes the pressure in the flow uneven. Under such conditions, not only is the gas density over the sample not uniform, but the distribution of plasma produced by discharge becomes nonuniform and unstable, the degrading of the uniformity of the etching speed. Hence, the gas flow speed should be set to below the sonic speed, or preferably one-third the sonic speed.
In an equipment configuration such as is shown in FIG. 16, wherein the gas inlet 104 is located near the side of the sample stage 110, which defines the gas outlet for the discharge zone 117, the gas flowing from the gas inlet 104 toward the discharge zone 117 is evacuated from the outlet before it can disperse in the entire space of the discharge zone 117. As a result, the gas is not effectively utilized. Moreover, depending on the shape of the discharge zone, the gas flow may fail to disperse sufficiently over the central area of the discharge zone 117.
Since only one gas flow rate controller 106 and one gas pipe 105 are commonly used for supplying one kind of gas, a deviation occurs with the gas flow and the discharge zone 117 with respect to the uniformity of etching.
A further problem with conventional systems is that, as the wafer diameter increases, it becomes increasingly difficult for the gas flow to disperse sufficiently to cover the central area of the discharge zone 117.